1. Technical Field of the Invention
The invention relates generally to acoustic sensors and more particularly to fiber optic acoustic sensors.
2. Prior Art
The use of laser interferometers for detection of acoustic signals is known. Typical prior art devices have been configured by winding an optical fiber onto a pressure sensitive mandrill or by suspending a pressure sensitive fiber in an acoustic medium, typically in a spiral configuration.
The limitations of the prior art devices include the requirement of suspending the array in a manner that does not restrict the acoustic response. This requirement greatly complicates suspension. Further, prior sensors are typically configured three-dimensionally, thereby further increasing suspension difficulties and limiting their use in moving flow fields. In the low wave number domain experienced by hull structures, pressure fluctuations in the turbulent boundary layer induce vibration in the hull structure resulting in vibrational noise. Prior sensors are sensitive to vibration induced noise and are therefore limited in overall sensitivity to acoustic energy signals due to the necessity of suppressing vibrational noise. Acoustic sensors used to quantify low wave number pressure components in a turbulent boundary have lacked sufficient sensitivity because of this required suppression of noise vibration induced. This lack of sensitivity is critical because the wavenumber characteristics of the turbulent boundary layer pressure fluctuations have large values at the structural wavenumber response of the hull of underwater vehicle. The hull structure in these circumstances is excited by the boundary layer pressure fluctuations and the resulting vibrations become a significant noise source. The vibration induced noise and impinging acoustic signals are in the same frequency range and therefore cannot be filtered by conventional means. Therefore, the presence of vibrational noise greatly reduces the effectiveness of prior hydrophone sensors in detecting and isolating acoustic energy signals from the acoustic medium.